1. Field of the Invention
The present invention generally relates to specific compounds which display enhanced moderate thermal stability, but will undergo thermal reactions at higher temperatures. It relates particularly to chemical compounds which contain a phenylethynyl group, which can be used as terminating species to transform multifunctional nucleophiles into reactive materials that can be converted thermally into thermosets. It also relates to compounds containing di(phenylethynyl) groups that can be used as reactive diluents.
2. Description of Related Art
Oligomers and monomeric compounds have been endcapped with various reactive species to generate materials that can be processed into various shapes with or without reinforcements. Upon thermal activation, these materials crosslink without the evolution of volatiles to form high molecular weight intractable resins. The reactive functionalities used to endcap various multifunctional compounds include epoxies, acetylenes, cyanates, and maleimides. An article entitled "Acetylene Terminated Prepolymers" by P. M. Hergenrother, Encycl. Polym. Sci. Eng., Vol. 1, 2nd Ed., John Wiley & Sons, Inc., New York, NY, (1985), p. 61, describes the use of ethynyl groups to endcap oligomers and the use of phenylethynyl groups along the polymer backbone.
More recently, B. Delfort et al., J. Polym. Sci., Part A., 28, 2451, (1990), and 29, 897, (1991) used 4-ethynyl-4'-fluorobenzophenone, 4-ethynyl-4'-fluorodiphenylsulfone, and their respective chlorinated and nitrated analogs to endcap hydroxy terminated arylene-ethers. These reactive oligomers were thermally treated at 160.degree. C. for 45 minutes then postcured at 250.degree.-270.degree. C. for 45 minutes to afford the arylene-ether thermosets. The reactive endcapping materials described in the Hergenrother article referred to above typically cure between 160.degree. C. and 250.degree. C. This limits both the thermal stability (pot life) of these systems at moderately elevated temperatures and the ability to decrease the melt viscosity at elevated temperatures. Also, these systems encounter problems of vitrification during the thermal curing cycle if the resulting material has a glass transition temperature (Tg) greater than the curing temperature. Thus, not only will these systems produce a material with a Tg no greater than that of the maximum curing temperature, but if the material has vitrified, it is unstable and a variation in the mechanical and thermal properties will occur throughout the life span of the material.